div.imgbox{
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
  div.mask{
    width: 100%;
    height: 100%;
    position: fixed;
    top: 0;
    left: 0;
    background: rgba(0,0,0,.6);
  }
  div:last-child{
    width: 600px;
    height: 400px;
    position: fixed;
    left: 50%;
    top: 50%;
    transform: translate(-50%,-50%);
    img{
      display: block;
      width: 100%;
    }
  }
}